The present application relates generally to controlling an economizer circuit in a vapor compression system. More specifically, the present application relates to controlling the economizer circuit of a vapor compression system by controlling a valve for the economizer port of a compressor.
In vapor compression systems such as refrigeration and chiller systems, a refrigerant gas is compressed by a compressor and then delivered to a condenser. The refrigerant vapor delivered to the condenser enters into a heat exchange relationship with a fluid, e.g., air or water, and undergoes a phase change to a refrigerant liquid. The liquid refrigerant from the condenser flows through a corresponding expansion device(s) to an evaporator. The liquid refrigerant in the evaporator enters into a heat exchange relationship with another fluid, e.g. air, water or other process fluid, and undergoes a phase change to a refrigerant vapor. The other fluid flowing through the evaporator is chilled or cooled as a result of the heat exchange relationship with the liquid refrigerant and can then be provided to an enclosed space to cool the enclosed space. Finally, the vapor refrigerant in the evaporator returns to the compressor to complete the cycle.
To provide increased capacity, efficiency and performance of the refrigeration or chiller system, an economizer circuit can be incorporated into the system. An economizer circuit can include an economizer heat exchanger or flash tank, an inlet line to the economizer heat exchanger or flash tank that is connected to the condenser or to the main refrigerant line downstream of the condenser, and an economizer expansion device, which is incorporated in the inlet line. When the economizer circuit includes a flash tank, a first outlet line from the flash tank can be connected to the main refrigerant line upstream of the expansion device, and a second outlet line from the flash tank that can be connected to a port within the compression chamber of the compressor or to the suction inlet of the compressor.
In flash tank economizer circuits, liquid refrigerant from the condenser flows through the inlet line and expansion device into the flash tank. Upon passing through the expansion device, the liquid refrigerant experiences a pressure drop, whereupon, at least a portion of the refrigerant rapidly expands or “flashes” and is converted from a liquid to a gas. The liquid refrigerant in the flash tank collects at the “bottom” of the flash tank and returns to the main refrigerant circuit through the first outlet line. The first outlet line may incorporate one or more valves to control the amount of liquid refrigerant returned to the main refrigerant circuit. The gaseous refrigerant in the flash tank collects at the “top” of the flash tank and returns to the compressor through the second outlet line to either the suction inlet or a point in the compression chamber operating at an intermediate pressure. The second outlet line may also incorporate one or more valves to control the amount of gaseous refrigerant provided to the compressor.
As discussed above, an economizer circuit can be used to provide increased capacity, efficiency and performance of the refrigeration or chiller system. For example, the economizer circuit can improve system efficiency by providing refrigerant gas at an intermediate pressure to the compressor, thereby reducing the amount of work required by the compressor and increasing compressor efficiency. A variety of parameters in the economizer circuit can be controlled to provide the increased capacity, efficiency and performance of the refrigeration or chiller system. The amounts of refrigerant entering and leaving the flash tank can be controlled, as well as the amount of liquid refrigerant maintained in the tank, to obtain the desired capacity, efficiency and performance of the refrigeration or chiller system.
There are two basic types of economizers that can be used in a refrigeration or chiller system. The first type of economizer uses a flash tank to cool refrigerant liquid by boiling a portion of the refrigerant and providing sufficient space to separate the liquid and gas phases. The cooled refrigerant liquid continues to an evaporator and the refrigerant vapor goes into the compressor. A solenoid valve can be used to regulate the flow on the vapor line between the flash tank and the compressor. A description of a flash tank economizer is described in U.S. Pat. No. 7,353,659, which patent is incorporated herein by reference. A second type of economizer uses a heat exchanger with subcooled refrigerant liquid on one side and boiling refrigerant on the other side. An expansion valve modulates the flow of the liquid refrigerant on the boiling side of the heat exchanger. The expansion valve can be controlled to maintain a constant superheat of refrigerant vapor leaving the heat exchanger. In other cases, the expansion valve can be controlled to maintain a constant compressor suction pressure or cooling capacity.
One problem with refrigeration or chiller systems involves the use of a variable speed drive to reduce compressor speed in response to high compressor motor current conditions. The problem is that reducing the frequency of voltage supplied to the compressor motor does not reduce the motor current for a given condensing temperature. Relatively large reductions in motor speed, which is related to the supply frequency from the variable speed drive, are required to reduce the condenser load and thereby reduce the motor current. The motor speed approach for compressor unloading results in a much larger reduction in cooling capacity than would be required with other techniques such as slide valve unloading in a screw compressor.
Therefore, what is needed is a system and method to control motor current while still maintaining a desired amount of cooling capacity. More specifically, what is needed is a system and method for simply and easily controlling an economizer circuit to provide improved performance to a refrigeration or chiller system while controlling motor current.